1. Field of the Invention
The present invention relates to a power conversion controller, and more particularly to a power conversion controller capable of providing PFC (Power Factor Correction) for power conversion applications.
2. Description of the Related Art
FIG. 1 illustrates a power conversion application, in which a prior art PWM/PFC circuit capable of providing a PWM (Pulse Width Modulation) function and a power factor correction function is used to control a BUCK conversion such that the waveform of an input current of the BUCK power conversion application is analog to that of a full-wave rectified input voltage VFull—Wave, and a regulated DC output voltage VO (or output current IO) is generated. As can be seen in FIG. 1, the power conversion application includes a PWM/PFC circuit 100, a power switch 101, a diode 102, an inductor 103, and a load 104.
The PWM/PFC circuit 100 is used to generate a driving signal VG according to a feedback signal VFB and the full-wave rectified input voltage VFull—Wave, so that the output voltage VO or output current IO is regulated at a DC value with the input current IIN following the full-wave rectified input voltage VFull—Wave, wherein the feedback signal VFB is generally derived from the output voltage VO or from the input current IIN.
The power switch 101, typically a MOSFET, is used to control the power conversion from the full-wave rectified input voltage VFull—Wave to the output voltage VO.
The diode 102 is used as a unilateral switch to release the energy stored in the inductor 103 when the power switch 101 is turned off.
The inductor 103 is used to store energy in the form of current when the power switch is turned on, and release the stored energy to the load 104 when the power switch 101 is turned off.
The load 104 can be a resistive load or a non-resistive load composed of LEDs.
When in operation, due to the full-wave rectified waveform, the voltage level of VFull—Wave falls below the output voltage VO during part of a period, and no energy is transformed from VFull—Wave to VO during that part of a period. As a result, the energy delivered to the load 104 in a period is dependent on the amplitude of VFull—Wave, the higher the amplitude, the more energy delivered in a period. Please refer to FIG. 2, which illustrates the waveforms of a high amplitude full-wave rectified input voltage VFull—Wave.High and a low amplitude full-wave rectified input voltage VFull—Wave.Low compared with a DC output voltage VO. As seen in FIG. 2, VFull—Wave.High has a dead time TdH during when VFull—Wave.High is lower than VO, and an active time TaH during when VFull—Wave.High is higher than VO, and VFull—Wave.Low has a dead time TdL during when VFull—Wave.Low is lower than VO, and an active time TaL during when VFull—Wave.Low is higher than VO, wherein TdH is shorter than TdL and TaH is longer than TaL. Therefore, the energy delivered in a period by VFull—Wave.High is more than that by VFull—Wave.Low, and it causes inconsistency in many aspects—such as power factor, average output current, etc.—of the power conversion application. For example, if the output current IO is to be regulated at a DC value ICONST, then the resulted average of the output current IO will be equal to ICONST×TaH/(TaH+TdH) for the high amplitude full-wave rectified input voltage VFull—Wave.High, and equal to ICONST×TaL/(TaL+TdL) for the low amplitude full-wave rectified input voltage VFull—Wave.Low, and it can be shown that TaH/(TaH+TdH) is larger than TaL/(TaL+TdL).
In view of this inconsistency problem, the present invention proposes a PFC power conversion controller capable of fixing the dead time for power conversion applications.